I'm rather surprised that nobody has mentioned the Phoenix Wright soundtracks. I honestly think that the games would not have been nearly as good if not for the BGM. The music is what made the game compelling. The gameplay made it interesting, the writing made it entertaining, but the music was the heart of the game. From the light-hearted and fun "Won the Case" to the intense court-battle themes like "Pursuit" and "Objection!" (Ooh, original name there.) the character themes just right, bringing so much life to each person you met. That is one of the few soundtracks that I would consider to be integral to the game itself. Most games have great music, but rarely is that music a real part of the gameplay experience, indispensable.